Generally, Gasoline Direct Injection (GDI) engines, unlike Multi Point Injection (MPI) engines, introduce lots of EGR gas into a combustion chamber in order to reduce the burden of a catalyst for removal of nitrogen oxide. This is so that the amount of nitrogen oxide contained in burnt gas can be reduced.
However, there is a problem in the GDI engines thus described in that an engine runs under a super thin air state such that EGR gas introduced into a combustion chamber is less likely to combust properly. Particularly, a mixture level between fresh air and the EGR gas greatly influences the combustion stability for GDI engines.